(update) In the last 5 days, this has been happening a lot, sometimes 5 or 6 games in a row. If you see artifacts pop up on the loading screen, hit the window key asap and go to your desktop. Wait ten seconds or so, then go back to L4D2. The artifacts should be gone. This has worked the last 3 times I've tried it, your mileage may vary. I've made a habit of hitting the windows key during the first loading screen for each game join (whether I see artifacts or not), and the issue has resolved for me.